Recently observation methods using near infrared light are increasing in research fields using biological microscopes. This is because of the advantage that a deep area that cannot be seen with visible light can be observed, since scattering in biological tissue is less, and photo-toxicity is low. For such observation methods, various applications, including 2-photon, CARS and SHG, are known. Even in conventional fluorescent observation methods, those using fluorescence dye, to cause excitation using longer wavelengths, are increasing in terms of photo-toxicity. On the other hand, observation methods using shorter wavelengths for photo excitation are still in high demand because of its high fluorescent efficiency (e.g. laser with about a 405 nm wavelength used for excitation light source for photo activation).
Considering such a situation, the performance demand for microscope objective lenses is that various aberrations are corrected throughout a wider wavelength area, and in particular that out of focus rarely occurs at each wavelength, and a sufficiently small spot diameter is obtained at each wavelength. An extension of the working distance is also desired in order to improve operability.
Thus far, an objective lens, in which chromatic aberration is corrected well throughout a very wide wavelength area of the g-line to t-line, has been disclosed (e.g. see Japanese Patent Application Laid-Open No. 2006-65030).